Methods of reducing nitrogen oxides in exhaust gases are known. The DE-OS-3615021 describes a method for the selective catalytic reduction of nitrogen oxides from exhaust gases of internal combustion engines by adding ammonia in a reactor. In accordance with this method the addition of ammonia is effected in dependence on the NO.sub.x -concentration in the exhaust gas, and the NO.sub.x -concentration is determined indirectly by measuring operating parameters of the internal combustion engine and subsequently calculating the concentration of nitrogen monoxide and nitrogen dioxide in dependence on at least one selected operating parameter on the internal combustion engine in consideration of families of characteristics.
In Rompps Chemie-Lexikon, 8th edition, pp. 1484 to 1490 the operation of glass-melting furnaces is described in detail. Glass-melting furnaces mostly are tank furnaces having a plurality of laterally disposed burners which are operated in alternation. The actual heating of the glass-melting furnaces is mostly effected by means of long-distance gas, heating oil or natural gas. The exhaust gases produced contain nitrogen oxides, due to fuels, high temperatures or additives. During the denitrification of exhaust gases, the NO.sub.x -content of the pure gas must, for legal reasons, always be monitored in connection with the O.sub.2 -content of the pure gas, which leads to the fact that in practice the setpoint of the NO.sub.x -content, NO.sub.x set, is transformed into a standardized setpoint NO.sub.x set n. In general, the following relation is used for the standardization: ##EQU1##
However, this standardization is disadvantageous when the glass-melting furnaces comprise several burners which are operated in alternation. If one burner is switched off during a combustion break, the NO.sub.x -content of the exhaust gas drops to a relatively large extent. When regulating the content of NO.sub.x in the exhaust gases by means of a simple regulator circuit, the introduced amount of NH.sub.3, which reacts with the nitrogen oxides in a known manner, is dependent on the deviation xd, wherein: EQU xd=NO.sub.x setn-NO.sub.x '
With decreasing NO.sub.x -content of the exhaust gases both the value NO.sub.x set n and the value NO.sub.x ' are decreased, which leads to the fact that the deviation xd does not or only insignificantly change. Since with a reduction of the content of NO.sub.x in the exhaust gases the deviation xd changes only insignificantly, the amount of NH.sub.3 to be supplied likewise remains almost constant in the denitrating plant, which leads to the fact that more NH.sub.3 is introduced than can be reacted with the nitrogen oxides. This in turn leads to the fact that the content of NH.sub.3 in the pure gas generally exceeds the admissible limit values. A further disadvantage of this conventional known regulation lies in the fact that the denitrating plant is generally not arranged in direct vicinity of the glass-melting furnaces. Thus, the exhaust gas requires some time to flow from the glass-melting furnace to the pure-gas port of the denitrating plant, in which port the pure gas values are measured in general. When the operation of a burner is interrupted, a NO.sub.x -content is measured in the denitrating plant which requires a higher amount of NH.sub.3 than this is actually necessary with the real values in the glass-melting furnace. Thus, a certain time must elapse before a regulation by means of a simple regulator circuit can be performed to react on the individual combustion breaks of the burners in the glass-melting furnaces.